1. Field of the Invention
The present invention relates to a vibratory meter and method, and more particularly, to a vibratory meter and method for determining a resonant frequency.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the conduit and the material therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things.
Vibratory meters, such as Coriolis mass flow meters and vibratory densitometers, may locate and measure a resonant frequency of a vibrating flow conduit or conduits. The resonant frequency can comprise a resonant frequency of the empty flow conduit or conduits or can comprise a resonant frequency of a fluid-filled vibratory meter. The flow material can be flowing or stationary. The measured vibrational frequency of the empty flow conduit(s) may be taken into account when processing the measured resonant frequency of the fluid-filled vibratory meter, in order to obtain the density of the fluid alone.
The resonant frequency may be used to determine the density (ρ) of the flow material. The density may be determined from ρ=C(1/f)2, where f is the measured resonant frequency and C is a calibration constant. In addition, the resonant frequency may be used in determining a mass flow rate of the flow material and may be useful in generating other fluid characteristics.
The flow material can comprise any manner of fluid, including liquids, gases, or mixtures of liquids, gases, and/or solids. Because gases have much lower densities than liquids, any error in measured resonant frequency will affect gas density measurements far more than an error will affect liquid density measurements. In addition, a small frequency error may translate into a much larger gas density error than for a liquid density measurement.